The present invention relates to a muffler, preferably for an internal combustion engine.
A known muffler of this type is disclosed in, for instance, Japanese Patent Unexamined Publication No. 60-27719. This muffler is arranged such that the interior of the muffler is partitioned into a plurality of chambers communicating with each other through an inner pipe, and this inner pipe is provided with an opening and closing valve which opens and closes in correspondence with the engine rotational speed. Thus, the opening and closing of the valve is controlled in accordance with given objectives. For instance, when the engine rotational speed is normal, the opening and closing valve is opened to reduce the internal pressure of the muffler, thereby reducing fuel consumption. When the engine rotational speed is low, emphasis is placed on the sound-damping effect rather than on back pressure, so that the opening and closing valve is closed. On the other hand, when the engine rotational speed is high, emphasis is placed on a high output through reduced back pressure rather than on the sound-damping effect, so that the opening and closing valve is opened.
In addition, the aforementioned publication also discloses a muffler in which, as shown in FIG. 6, a plate valve 3 is provided slidably in an inlet pipe 2 disposed in an outer shell 1, and the plate valve 3 is urged by a spring 4. In a medium-speed region, the plate valve 3 is closed by the urging force of the spring 4, while, in a high-speed region, the plate valve 3 is closed by the flow of an exhaust gas against the urging force of the spring 4.
Furthermore, the same publication discloses another muffler which is arranged such that, as shown in FIG. 7, an opening and closing valve 6 is provided on the exhaust port side of an inner pipe 5 within an outer shell 1 in a face-to-face relationship, and this opening and closing valve 6 is moved forward and backward by a moving member 7 so as to open and close the inner pipe 5.
With the above-described structures, however, there is a problem in that when the opening and closing valve 3 or 6 opening the flow of the exhaust gas is restricted by coming into contact with the entire opposing surface of the opening and closing valve 3 or 6, and the back pressure therefore becomes high.